It is known that droplets with a volume of more than 10 μl can be dispensed from the air very easily, since if the pipette is correctly manipulated, the droplets leave the pipette tip of their own accord. The droplet size is then determined by the physical properties of the sample liquid, such as surface tension or viscosity. The droplet size thus limits the resolution of the quantity of liquid to be dispensed.
The aspirating and dispensing, i.e. the pipetting of liquid samples with a volume of less than 10 μl, in contrast, typically requires instruments and techniques which guarantee the dispensing of such small samples. The dispensing of a liquid with a pipette tip, i.e. with the endpiece of a device for aspirating and/or dispensing sample liquid, can occur from the air (“from air”) or by touching a surface. This surface can be the solid surface of a container (“on tip touch”), into which the liquid sample is to be dispensed. It can also be the surface of a liquid in this container (“on liquid surface”). A mixing procedure following the dispensing is recommended, particularly for very small sample volumes in the nanoliter or even picoliter range, so that uniform distribution of the sample volume in a diluent is ensured.
Disposable tips significantly reduce the danger of unintentional transfer of parts of the sample (contamination). Simple disposable tips are known (so-called “air-displacement tips”), whose geometry and material is optimized for the exact aspirating and dispensing of very small volumes. The use of so-called “positive-displacement tips”, which have a pump plunger inside, is also known.
For automation of the pipetting process, two procedures must be differentiated from one another: the defined aspiration and the subsequent dispensing of liquid samples. Between these procedures, typically the pipette tip is moved by the experimenter or by a robot, so that the aspiration location of a liquid sample is different from its dispensing location. For precision of aspiration and dispensing, only the liquid system is essential, which includes a pump (e.g. a diluter implemented as a syringe pump), tubing, and an endpiece (pipette tip). Among the many possible pumps for highly precise aspirating and dispensing of liquids, commercially available devices with the name “CAVRO XL 3000 Modular Digital Pump” or “CAVRO XP3000 plus Modular Digital Pump”, sold by the firm Cavro Scientific Instruments Inc., Sunnyvale, Calif. USA, are available and perform suitably. Such pumps typically include a cylinder with a piston movable therein and a stepping motor for driving the piston. The stepping motor operates at a voltage of 24 V and is controlled by an external computer or microprocessor. Further details can, for example, be found in the “Operators Manual P/N 724043C” from Cavro Scientific Instruments Inc.
A device and a corresponding method for dispensing volumes are known from U.S. Pat. No. 5,763,278. This reference discloses automatic pipetting of small volumes, having a device including a pipetting needle, a diluter with a liquid outlet having a syringe, and a valve. The syringe includes a piston and a piston drive. A line connects the needle and the liquid outlet of the diluter, with the diluter and the line containing an essentially incompressible liquid. A pulse generator is located in the device and connected with the incompressible liquid in the line so that mechanical pulses with a force of at least 0.01 Ns can be output directly into the liquid of the line. A pulse of this type serves for driving the liquid out of the needle. The droplet size is defined by a targeted advance of the diluter piston and the droplet is ejected from the needle with a pulse. By defining the volume with the diluter, the droplet size and the reproducibility thereof depends on and is limited by the resolution of the diluter.
A pipetting device of this type that includes a piston pump and a pulse generator in the form of a piezoelectric element is disclosed in Japanese patent application JP 09 327628. The piezoelectric element is also the front plate of the piston and is used for terminating the dispensing procedure. The piston effects the majority of the liquid dispensing by its downward movement and is blocked during the actuation of the piezoelectric plate. The movement direction of the piezoelectric plate corresponds in this case to that of the piston. At least a part of the volume dispensed thus always depends on the movement of the piston, so that the reproducibility of the piston movement limits the resolution of the pipetting device.